1. Field of the Invention
The invention relates to micro fuel cells, i.e., fuel cells useful in small portable devices such as laptop computers and cellular phones.
2. Discussion of the Related Art
Powering of portable electronic devices is a significant issue in today""s marketplace, particularly with the increasing use of wireless technologies. While the speed and functionality of many portable telecommunications and computing devices tend to be limited by the power sources, the availability of good power sources is lagging behind development of the electronic devices themselves. Thus, improved power supply and management is constantly being sought.
One of the primary factors for power supply, particularly for portable computers, is the lifetime of the battery between charges. Consumers seek a useful lifetime between charges, e.g., sufficient to power a laptop computer for the duration of a train trip or a plane ride. Yet, consumers also tend to be annoyed by batteries"" weight, lengthy recharge times, and expense. Moreover, some types of rechargeable batteries, e.g., nickel-cadmium, contain environmentally undesirable materials and will therefore soon be prohibited from import into Europe. Because of these issues with batteries, some efforts to improve power supply and management have been directed at developing miniaturized fuel cells.
Conventional proton exchange membrane fuel cells, e.g., of the type used for automotive applications, operate as shown schematically in FIGS. 1A and 1B. FIG. 1A shows an exploded view of the cell 10. The cell 10 contains an anode current collector 12 and a cathode current collector 14, both typically formed from a graphite block with machined paths 13, 15 for directing fuel or an oxidant. Graphite cloths 16, 18 are provided to allow for gas diffusion from the current collectors 12, 14 to a centrally-located proton exchange membrane 20 having catalyst films, typically platinum, formed on each side.
As shown in the cross-sectional schematic of FIG. 1B (not to scale), the cell 10 is put together as a sandwich structure. Fuel, e.g., hydrogen gas, moves through the machined paths in the anode current collector 12, diffuses through the graphite cloth 16, and contacts the catalyst layer 17. The catalyst strips electrons from the fuel, the electrons then traveling through external circuit 22. The remaining positive ions travel through the membrane 20 to a second catalyst layer 19, wherein they combine with oxygen ions formed when the free electrons travel from the circuit 22 and combine with an oxidant fed through the machined channels of the cathode current collector 14. The by-products of the process are heat, water, and the electricity generated by the electron flow.
While a body of research exists for large-scale fuel cell stacks that generally provide 10,000 to as high as 250,000 Watts, portable electronic devices require only 0.5 to 20 W. Thus, the existing fuel cell technology has not been designed or optimized for miniaturized fuel cells for portable electronic devices. And efforts to develop fuel cells on this smaller scale have not yet led to any design proven to be feasible and commercially-acceptable.
For example, one effort at an improved design is reflected in J. D. Morse et al., xe2x80x9cA Novel Proton Exchange Membrane Thin-Film Fuel Cell for Micro-scale Energy Conversion,xe2x80x9d presented at American Vacuum Society meeting, Oct. 4, 1999. Morse et al. utilized silicon as a support structure, and photolithographically defined patterned gaps in the silicon for directing fuel to the anode (see FIG. 1 of Morse et al.) After patterning this silicon, the group followed conventional techniques, and formed on the silicon a nickel anode current collector, with gas diffusion holes etched therein (which they refer to as xe2x80x9cporousxe2x80x9d), a platinum catalyst layer (not shown), an electrolyte layer, a platinum cathode contact layer (not shown) and a silver cathode current collector. While this work is interesting, the results show a relatively inefficient fuel cell that does not suggest a commercially applicable design. Moreover, the nickel and silver in the cell will be subject to attack, and would thereby be expected to dissolve in a relatively short time period.
Thus, improved designs for miniature or micro fuel cells useful for portable electronic devices are desired.
The invention relates to improved micro fuel cells suitable for portable electrical devices, and a processes for forming the fuel cell. In one embodiment of the invention, silicon substrates are used both as the gas delivery structure for the fuel and the oxidant, and as the current collectors. Such use of silicon is advantageous in that it becomes possible both to utilize micromachining and lithographic techniques to form the desired structures, e.g., the gas delivery channels, and also to integrate the fuel cell with silicon-based control circuitry. In addition, by using silicon substrates as both current collectors and gas delivery structures, the invention achieves a simpler, smaller fuel cell, in contrast, e.g., to the Morse et al. fuel cell, supra, in which the silicon is only a structural member on which a separate metal anode current collector must be formed. In addition to the silicon substrates, the fuel cell of this embodiment of the invention further contains a porous gas diffusion region or regions overlying the gas delivery tunnels, a catalyst layer, and a proton exchange membrane. (As used herein, the term layer indicates either a continuous or a discontinuous, e.g., patterned, layer.)
In one particular aspect of this first embodiment, reflected in FIG. 2D, the silicon substrates comprise both gas delivery tunnels 36 and porous silicon gas diffusion regions 32 formed over the tunnels in the surface of the substrate, i.e., the porous regions over the gas delivery tunnels are integral with the silicon substrate. These channels are generally formed by electrochemically etching the silicon substrate to first create the porous silicon regions along its surface, and then, by changing the processing conditions to move into an electropolishing regime, polishing out the underlying tunnels while the porous regions are left intact. The resultant structure is a porous silicon layer suspended over the tunnel regions. These porous silicon gas diffusion regions are advantageous in that they provide a large surface area over which gas diffusion from the tunnels onto the catalyst layer occurs, e.g., as compared to use of a metal or other film with holes etched therein.
In another embodiment of the invention, reflected in FIG. 4, a monolithic structure is employed. In this structure, in contrast to the sandwich-type structure of the previous embodiment, only a single silicon substrate is required, and this substrate does not act as a current collector. Specifically, as shown in the Figure, the substrate contains independent gas delivery tunnels 84, 86 for the fuel and for the oxidant, a catalyst layer 88, 89, distinct cathode and anode regions, 90, 92, and a proton exchange membrane 94. Benefits of this monolithic structure include simpler fabrication and, because of the exposed surface, improved control of the system hydration.